Problem: Factor the following expression: $4$ $x^2$ $-1$ $x$ $-18$
Explanation: This expression is in the form ${A}x^2 + {B}x + {C}$ . You can factor it by grouping. First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(4)}{(-18)} &=& -72 \\ {a} + {b} &=& & & {-1} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $-72$ and add them together. Remember, since $-72$ is negative, one of the factors must be negative. The factors that add up to ${-1}$ will be your ${a}$ and ${b}$ When ${a}$ is ${-9}$ and ${b}$ is ${8}$ $ \begin{eqnarray} {ab} &=& ({-9})({8}) &=& -72 \\ {a} + {b} &=& {-9} + {8} &=& -1 \end{eqnarray} $ Next, rewrite the expression as ${A}x^2 + {a}x + {b}x + {C}$ $ {4}x^2 {-9}x +{8}x {-18} $ Group the terms so that there is a common factor in each group: $ ({4}x^2 {-9}x) + ({8}x {-18}) $ Factor out the common factors: $ x(4x - 9) + 2(4x - 9) $ Notice how $(4x - 9)$ has become a common factor. Factor this out to find the answer. $(4x - 9)(x + 2)$